Converters having a voltage intermediate circuit and an inverter downstream from the voltage intermediate circuit are known from the printed publication EP 0 899 859 B1. The inverter supplies a three-phased alternating voltage at its outputs. A filter system connects the output terminals of the inverter and the voltage intermediate circuit. The filter system includes a filter choke connected in series with the output terminals and a filter capacitor which leads from each output terminal to a common capacitor star point.
EP 0 899 859 B1 teaches in paragraph [0006] that only the pure three-phase voltage systems are filtered. If one were to connect the capacitor star point directly to a center tap located between two intermediate circuit capacitors, then the direct voltage system is indeed filtered as well, but disadvantages result with regard to the system technology. Without additional measures, the filter system is constantly excited to natural oscillations. As taught in paragraph [0007] of EP 0 899 859 B1, only a regulation that intervenes in the switching sequence of the inverter can damp these natural oscillations without losses.
These natural oscillations are caused by the series resonant circuit made up of the filter chokes and the filter capacitors that are connected to the common capacitor star point.
According to paragraph [0007] of EP 0 899 859 B1, the switching sequence of the inverter is nowadays usually defined by a control computer, which determines the switching commands for the power switches based on the load conditions. The control computer normally operates in only two coordinates because of the symmetry of the output, from which the switching states of the three phases are able to be determined. The three phases are decoupled in the filter circuit. A resonance regulation thus must be performed using three independently acting control circuits. The resonance regulation thus is no longer easily able to be integrated into the load regulation. However, since both parts of the control access the same control variable, they cannot operate in a decoupled manner.
Also disadvantageous in the afore-described circuit of EP 0 899 859 B1 is the high additional current loading of the intermediate circuit capacitors and the inverter, which results especially at low switching frequencies of the inverter-semiconductor switches from the charge reversal of the filter capacitors by the direct voltage system.
As disclosed in FIG. 5 of EP 0 899 859 B1, the capacitor star point is connected to the voltage intermediate circuit via an RC element formed by a resistor, which is connected in series with a capacitor.
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